1. Field of the Invention
This invention relates to vehicle electrical systems in which more than one DC voltage is required. More particularly, this invention relates to DC to DC converters used in the power management system of multiple voltage vehicles for managing the power available from the vehicle alternator and the battery, and for directing and redistributing the available power to loads requiring different DC voltages.
2. Description of Related Art
Most vehicles that are presently available operate with a nominal 12 volt DC electrical power system to run the various accessories, fans, wipers and other electrically operated components. The number of components and the power needed to operate them has been increasing and it is expected that this trend will continue. A new generation of engines which use electromagnetically operated engine valves to improve vehicle efficiency will require even more electrical power.
When supplying high power at 12 volts, the current carried by the vehicle wiring is significant, and this high current results in a substantial resistive power loss through the wiring. To minimize this loss, it is possible to decrease the wiring resistance by increasing the diameter of the vehicle wiring, but large diameter wiring is expensive. Large diameter wiring also consumes valuable space within the vehicle and can be difficult to install. For all of these reasons, there has been an increased interest in using higher DC voltages in vehicles.
Although conventional vehicle power systems are commonly referred to as 12 volt systems, those skilled in this art will recognize that the actual voltage provided by these systems is about 14 volts when the vehicle alternator is turning. The 14 volt output of the alternator allows the nominal 12 volt battery to be fully charged. Higher voltage systems that are most commonly considered use multiples of the basic 12/14 volt system, such as a multiple of three forming a 36/42 volt system or a multiple of four providing a 48/56 volt system, etc. Hereafter, these dual labels of 12/14, 36/42, 48/56 volts, etc. will be used where appropriate.
Although higher voltage systems have significant advantages for high power consuming components, for low power consumption accessories the conventional 12/14 volt DC power is more suitable. In order to remain compatible with the present industry standard 12/14 volts, it is contemplated that 12/14 volt electrical systems will be required in all vehicles in addition to the higher DC voltage system.
It is also desirable for the vehicle to be able to supply conventional 110 volt AC. This allows household appliances to be operated in the vehicle and for the vehicle to be used as an emergency power source.
One approach to providing multiple DC voltages is to use a separate battery for each DC voltage needed. Multiple batteries are heavy, and relatively expensive, however, as compared to the weight and cost of current single battery systems.
Another disadvantage of multiple batteries is that each battery requires an independent and reasonably precise voltage control system to ensure rapid and complete recharging of the battery while avoiding battery damage due to overvoltage or excessive discharge. Separate alternators can be used to meet this need for charging each battery and regulating the charging voltage, but the significant cost and complexity makes this approach unfeasible.
A single alternator with multiple output windings, one for each voltage, may also be considered for producing the multiple voltages for multiple batteries. However, such a design must address the problem of uncorrelated variable loads on each battery at the different voltages. It is generally not possible to provide precise enough voltage regulation for each separate battery by adjusting the current to a single field winding in such an alternator. Any individual battery could be controlled precisely, or a weighted sum of the separate battery voltages could be regulated precisely, however, unbalanced loading would lead to poor battery voltage control and shortened battery life if limited to a single alternator and regulator.
Multiple voltage systems employing a single multiple output winding alternator can be used in single battery power systems as the output winding supplying the battery voltage can be carefully regulated by controlling the alternator field winding. The more modest regulation on the other outputs would normally be acceptable if no batteries are present on these outputs. A difficulty can arise in this type of system, however, if multiple voltages are required when the engine is off. In this situation, only the single battery voltage would by available prior to engine start.
This is particularly a problem with the latest high efficiency vehicle engine designs which use electromagnetically operated engine valves. These valves must be powered in order to start or run the engine. Electromagnetically driven engine valves consume power at a rate which is roughly proportional to the vehicle engine speed. At low speeds as the engine is started, these valves consume relatively little power and consume progressively more as the engine speed increases. Multiple voltage alternator systems for vehicles have not previously been designed to match this characteristic requirement for electromagnetically operated engine valves.
A related difficulty is that utilization of multiple winding alternators can lead to alternator inefficiencies. This comes about because the alternator is not operated in the most efficient manner unless the power loads on each winding track each other. Unfortunately, the loads on the different voltages do not normally track each other. An alternator is most efficient if all the power is converted on a single three phase winding at the highest voltage as this minimizes diode losses.
An auxiliary converter can be utilized to create the other system voltages, however, such converters have generally employed power modulation techniques to provide the required regulation for multiple batteries. These power modulation techniques generally require power filtering elements such as chokes and aluminum electrolytic capacitors. Suitable filtering capacitors are also expensive and have temperature limitations making under the hood operation less feasible.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a multiple voltage alternator system requiring only a single battery.
It is another object of the present invention to provide a multiple voltage alternator system incorporating a simple DC to DC converter which does not require expensive filtering capacitors and where the vehicle starter is powered directly from the battery and does not require current supplied by the DC to DC converter.
It is another object of the present invention to provide a multiple voltage alternator system having a nominal 12/14 volts DC as one of the output voltages so that the extensive 12/14 volt vehicle equipment currently in use is not made obsolete.
A further object of the invention is to provide a multiple voltage alternator system which is capable of automatic DC to DC voltage conversion wherein power flows bi-directionally through the DC to DC converter from the low voltage side to the high voltage side and back as necessary depending upon the load requirements for each voltage.
It is yet another object of the present invention to provide a multiple voltage alternator system in which a conventional low voltage battery can be used during starting and for supplying initial power to electromagnetically operated valves in a vehicle engine and in which the alternator output windings are designed for higher voltage operation and provide higher electromagnetic engine valve power at a higher voltage as the vehicle engine speed increases and more power is required.
Yet another object of the present invention is to provide a multiple voltage alternator system that can provide 110 volt 50/60 Hz AC power.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.